More than seven months since The Year of Magical Thinking finished its run on Broadway, Vanessa Redgrave is excited to be returning to the play at the National Theatre.

"It is always wonderful coming back after a good big gap," she says.

"The only analogy I can think of is when in your life you reconsider some period and suddenly see it in a different light, so you are seeing things you didn't see at the time which can only be beneficial. Whether one likes it or not." She laughs a little as she says this, in her grave, soft voice. In person she is smaller and more graceful than I had imagined; she speaks slowly with a weight of thought behind her words.

It is this quality of intelligence as much as her charisma that made Redgrave such obvious casting. But she is insistent that she is not impersonating Didion.

"I remember saying to David Hare, 'I'm not supposed to be playing her, am I, because I don't see how I could possibly?'

And he said: 'No way.'"

She is also clear that the fundamental difference between the book and the play is that Quintana, Didion's daughter, was alive when she was writing her memoir and had died by the time she came to shape the play.

"For me the whole motor force of the play is that Joan is in enormous fear and panic as to whether she will be able to save her daughter's life when her daughter's father isn't there to support her."

That rising sense of panic, the magical thinking that attempts to ward off death, has made the play an extraordinarily cathartic experience for audiences; in this sense, she says, it is not a soliloquy.

It is one character standing on stage, "talking to the audience," she says.

"They are the other characters. They are the point. They are why the play has been written." So the play helps people to deal with their own grief - however long ago they have experienced it. Redgrave says that she understands why some societies have extended periods for grieving and memorial; but our conventions often prevent it.

"Circumstances don't allow you to," she says, her voice dropping. "I have lost quite a lot of people very close to me and it was always at a time when I felt I couldn't break the commitments I had made."

When her mother, Rachel Kempson, died while she was performing Long Day's Journey Into Night on Broadway, she thought it was "monumentally understanding" that she was given three performances off. But does she draw on such experiences when she performs the play? Her answer is characteristically careful.

"You are not playing yourself, so the object of the exercise is to put yourself into someone else's shoes. But of course, stepping into someone else's shoes often kicks off memories. That is what happens to an audience in a play - they are assisted in stepping into other people's shoes. And if the play is really good, they start thinking about themselves in their own shoes. Which is the point of drama. Or one of the points, anyway."